This invention relates to a method for reducing carbon monoxide emissions from the regenerator utilized in catalytic cracking processes, such as the fluidized catalytic cracking process, by incorporating in the cracking catalyst a metal component active for the oxidation of carbon monoxide to carbon dioxide. More specifically, the invention relates to cracking catalyst compositions wherein rare earth components are incorporated into the cracking catalyst by spray impregnation.
In the petroleum industry, high boiling hydrocarbon feedstocks are charged to FCC units so that, by contact with a moving bed of catalyst particles, the feedstock is converted to a more valuable hydrocarbon product, such as gasoline, having a lower average molecular weight and a lower average boiling point than said feedstock. The most typical hydrocarbon feedstock so treated in FCC units consists of heavy gas oil, but on occasion such feedstocks as light gas oils, naphtha, reduced crudes, and even whole crudes are also subjected to catalytic cracking to yield low boiling hydrocarbon products.
Catalytic cracking in FCC units is usually accomplished in a cyclic process involving catalytic reaction, steam stripping, and catalyst regeneration. The hydrocarbon feedstock is blended with an appropriate amount of catalyst particles, and the mixture so produced is then passed through a catalytic reactor, commonly called a riser, wherein a catalytic cracking reaction zone is maintained such that at a temperature between about 800.degree. and 1100.degree. F. the feedstock is converted into gaseous, lower boiling hydrocarbons. After these lower boiling hydrocarbons are separated from the catalyst in a suitable separator, such as a cyclone separator, the catalyst, now deactivated with deposited coke is passed to a stripper. In the stripper, the deactivated catalyst is contacted with steam so as to convert some of the coke to hydrocarbon product vapors, which are then combined with the vapors received from the cyclone separator and transferred to other facilities for further treatment. Meanwhile, catalyst particles are recovered from the stripper, and because only a small proportion of the coke was removed in the stripper, the catalyst is introduced into a regenerator wherein, by combustion in the presence of an oxygen-containing gas such as air, the remaining larger proportion of the coke is removed and the catalyst reactivated. The cyclic process is then completed by again blending the reactivated catalyst particles with the feedstock entering the FCC unit.
One recognized problem in the conventional FCC process resides in the incomplete combustion in the regenerator. Due to the relatively low temperature of combustion in the regenerator, usually between about 900.degree. and about 1300.degree. F., the flue gas contains a substantial proportion of carbon monoxide in addition to carbon dioxide. Typically, the flue gas contains carbon dioxide and carbon monoxide in a ratio of 0.8:1 to about 10:1, CO.sub.2 :CO, with the carbon monoxide concentration often being as high as 15 mole percent. Such high concentrations of carbon monoxide are a source of concern inasmuch as it is a pollutant, and this concern has recently resulted in numerous methods to reduce the amount of carbon monoxide discharged from FCC regenerators and the like. In general, these methods aim to reduce the polluting effects of carbon monoxide by incorporating in the cracking catalyst particles metal components that are active for catalyzing the reaction of CO with oxygen at the low temperatures encountered in FCC regenerators. The metal components known for such use include the Group VIII metals and rhenium, as disclosed in U.S. Pat. No. 4,072,600, chromium, as described in U.S. Pat. No. 2,647,860, Group IB, IIB, VIB, VIIB, and VIII metals, as disclosed in U.S. Pat. No. 3,364,136, and certain rare earth metals, such as cerium, as taught in Netherlands Pat. No. 7300884 (GLADROW U.S. Pat. No. 3,823,092), and lanthanum, as taught in U.S. Pat. No. 4,137,151.
Of the foregoing metal promoters, platinum and the other noble metals are known to have the highest activity for oxidizing carbon monoxide. Ironically, however, such high activity is oftentimes undesirable and detrimental. In some FCC units, and especially those of older design, the high conversions of CO to CO.sub.2 attainable with a platinum-containing catalyst release heat in such large quantities that the temperature of the regenerator vessel is raised to levels that overheat the metals within the regenerator vessel. On the other hand, some metal promoters, such as rare earth promoters, usually do not sufficiently improve CO oxidation activity to justify the expense of incorporating them in cracking catalyst particles.
Accordingly, it is an object of the invention to provide a metal-promoted cracking catalyst of CO oxidation activity greater than that of prior art rare earth catalysts but less than that of the typical platinum-promoted FCC catalyst. It is another object to provide a method for increasing the CO oxidation activity of rare earth-promoted cracking catalysts by spray impregnating the rare earth components onto the cracking catalyst particles. These and other objects and advantages will become apparent in light of the following description of the invention and its preferred embodiment.